PROJECT SUMMARY/ABSTRACT Rapid weight gain during infancy has been recognized as a prime target for prevention and intervention efforts and formula-/bottle-feeding has been highlighted as one of the strongest modifiable predictors of risk for rapid weight gain during the postpartum period. Current recommendations and prevention programs aimed at reducing risk for rapid weight gain and obesity during early childhood have predominately focused on promoting responsive feeding practices, regardless of milk type or feeding mode used. However, previous research examining responsive feeding has taken a mother-centric approach, focusing on relating mothers' feeding practices and styles to infant outcomes without adequate consideration of how infant behaviors ? such as ability to effectively communicate satiation ? may influence feeding interactions. The study of dyadic interactions is common within the broader field of research on infant development, yet a marked paucity of research within the field of infant feeding attempts to objectively describe dyadic interactions during feeding to examine what both infants and mothers contribute to feeding interactions. The aim of the proposed research is to address this research gap by describing the co-development of infant communication of satiation and mothers' feeding practices across infancy and feeding contexts. The Co-Is are currently conducting the ABC Baby Study, an NICHD-funded (R01HD084163) prospective, longitudinal study of 325 mother-infant dyads assessed when infants were 2 weeks and 2, 4, 6, 9, and 12 months. The overall objective of the ABC Baby Study is to describe associations between developmental trajectories of infants' appetitive drives and weight gain. A unique feature of this study is that all dyads are video-recorded during naturalistic bottle-feeding interactions at each assessment and solid food feeding interactions at 6, 9, and 12 months. Yet, behavioral coding of the videos was not part of the parent grant aims. In the proposed study, we aim to add novel, objective measures of infant communication of satiation and maternal responsiveness to infant cues during feeding to allow for an innovative investigation that will inform future - and much needed ? primary prevention efforts. By combining existing study data with the novel data collected in the proposed research, we will aim to: 1) describe the dyadic interactions that occur during feeding at 2 weeks, and 2, 4, 6, 9, and 12 months and identify clusters of dyads who show distinct developmental trajectories of dyadic feeding interactions across infancy; 2) examine associations between cluster membership and risk for rapid weight gain between birth and 12 months; and 3) identify maternal and infant correlates of cluster membership. Findings from the proposed research will highlight modifiable mechanisms underlying associations between maternal-infant risk factors and infant outcomes, which will inform efforts to design tailored prevention programs best-fit to the needs of high-risk dyads.